Method for smoothing and polishing surfaces by treating them with energetic radiation

ABSTRACT

The present invention relates to a method for smoothing and polishing surfaces by treating them with energetic radiation, in particular laser radiation, in which the to-be-smoothed surface is remelted in a first treatment step using said energetic radiation and employing first treatment parameters at least once down to a first remelting depth of approx. 5 to 100 μm, which is greater than a structural depth of the to-be-smoothed structures of said to-be-smoothed surface, wherein continuous radiation or pulsed radiation with a pulse duration of ≧100 μs is employed. The method makes it possible to automatically polish any three-dimensional surface fast and cost effective.

TECHNICAL FIELD

The present invention relates to a method for smoothing and polishingsurfaces by treating them with energetic radiation, in particular laserradiation, in which the to-be-smoothed surface is remelted at least oncein a first treatment step, using the energetic radiation and firsttreatment parameters down to a first remelting depth which is greaterthan a structural depth of the to-be-smoothed structures of theto-be-smoothed surface and is ≦100 μm. The method can be used, forexample, in mechanical engineering for finishing tools and molds. Inmodern mechanical engineering and, especially, in tool and moldconstruction, there are high demands on the quality of technicalsurfaces. However, in manufacturing processes, such as for examplemilling or eroding, the achievable surface roughness is limited. Ifsmooth, glossy, respectively polished surfaces, are required, additionalmanufacturing processes, such as abrading and polishing, must follow.

PRIOR ART

Nowadays, tool and mold finishing occurs predominantly by means ofmanual polishing. Electrically or pneumatically driven devices with upto ultrasonic operating frequencies support the manual work. Until thefinal polishing step, the steps rough abrasion, fine abrasion andpolishing with increasingly finer polishing pastes up to diamond pastemust be carried out. Usual polishing times are about 30 min/cm2.Peak-to-valley heights of Ra<0.01 μm are achievable.

Mechanical polishing methods have the drawback that prior art methodscannot be applied or only yield unsatisfactory results if theto-be-polished surfaces have complex three-dimensional geometries.

DE 42 41 527 discloses a method of hardening machine components byheating the surfaces of the component using a laser beam, with which thesurface of the machine component can simultaneously be smoothed. In thiscase, the components are chilled cast parts with a ledeburitic structureor steel parts with a pearlitic structure. In this method, a surfacelayer of the component is heated using a laser beam until close to themelting point in such a manner that in a boundary layer, diffusion ofthe carbon occurs out of the cementite lamella of the ledeburite,respectively out of the pearlite, into the soft intermediate ferritelamella regions. The diffusion of the carbon results in the desiredhardening of the surface. Setting the laser parameters with energydensities in the range of 10³-10⁵ W/cm² results, moreover, in markedevaporation and melting of a thin surface skin, which simultaneouslyleads to micro-smoothing of the surface. This application requires thelaser power of approximately 4-12 kw.

Furthermore, EP 0 819 036 B1 describes a method using a laser to polishwith any three-dimensional mold surface in which the contour of theto-be-treated workpiece is first measured and then the treatmentstrategy and the treatment parameters are derived from the prescribeddesired shape and the current shape. Smoothing and polishing arerealized by a removal process. For laser polishing, a region of lowlaser intensity is proposed as greater material removal is not desiredin this application. However, there is no other mention in this printedpublication of treatment strategies or treatment parameters forachieving an optimum degree of smoothing. The heart of the proposedmethod is recognizing any deviation of the current shape from thedesired shape by scanning using a three-dimensional contour measuringdevice. From this deviation, the suited treatment parameters arecalculated and utilized. These steps are repeated until the desiredshape is obtained. However, the required use of a three-dimensionalcontour measuring device is complicated and, due to the precisiondemanded, connected with very high costs.

DE 197 06 833 A1 discloses a method for smoothing and polishing surfacesaccording to the generic part of claim 1. In this method, the surface isbrought to start to briefly melt with pulsed laser radiation having apulse duration of between 10 ns and 10 μs to a depth of 2 to 3 μm witheach laser pulse. The new molten mass generated with each laser pulsesolidifies again before the next laser pulse occurs. However, the methodis only suited for smoothing surfaces with a minimal surface roughnessof Rz≦3 μm.

The object of the present invention is to provide a method for smoothingand polishing surfaces by treating them with energetic radiation, inparticular laser radiation, and this method does not require expensivemeasuring instruments and can be used to automatically polish anythree-dimensional surface, in particular metal surfaces, quickly andinexpensively.

DESCRIPTION OF THE INVENTION

The object on which the present invention is based is solved using themethod according to claim 1. Advantageous embodiments of the methods arethe subject matter of the subordinate claims or can be drawn from thefollowing description and the preferred embodiments.

In the present method for smoothing, respectively polishing, surfaces bytreating them with energetic radiation, for example laser radiation orelectron radiation, the to-be-smoothed surface is remelted in a firsttreatment step using the energetic radiation and employing the firsttreatment parameters at least once down to a first remelting depth ofapproximately 5 to 100 μm, which is greater than a structural depth ofthe to-be-smoothed structure of the to-be-smoothed surface, usingcontinuous radiation or pulsed radiation with a pulse duration of ≧100μs.

Preferably a second treatment step is conducted then utilizing energeticradiation with second treatment parameters. In this second treatmentstep, the micro-roughness remaining after the first treatment step isleveled by remelting down to a second remelting depth, which is lessdeep than the first remelting depth, and by evaporating roughness peaks.This preferred embodiment of the present method is thus based on amulti-step treatment process, which can be divided into rough treatmentand fine treatment. In the first treatment step, also referred to in thefollowing as rough treatment, the to-be-smoothed surface is remeltedonce or multiple times down to a first remelting depth in a boundarylayer using the energetic radiation and employing the first treatmentparameters. In this remelting process, the macro-roughness which, forexample may stem from previous milling, respectively eroding process, isremoved. In a second treatment step, also referred to in the followingas fine treatment, the remaining micro-roughness on the surface is thenleveled using energetic radiation and employing the second treatmentparameters. The second treatment step of the fine treatment comprisestherefore a combination material removal and remelting process in whichthe thickness of the remelted boundary layer is less than the thicknessof the remelted boundary layer of the first treatment step.

With the proposed method, any three-dimensional workpiece surface can bequickly and inexpensively automatically polished. Measuring the contourof the to-be-polished surface is not required. Moreover, due to themulti-step treatment process with different first and second treatmentparameters, a high glossiness of the polished surface is achieved.

The method is particularly suited for smoothing three-dimensional metalsurfaces. For example, it has already been used to smooth and polishworkpieces made of the steels 1.2343, 1.2767 and 1.2311 as well as oftitanium materials. Of course, the present method can also be utilizedwith other metals and non-metals such as, for example, workpieces madeof plastic. Someone skilled in the art needs only to adapt the treatmentparameters to the to-be-treated materials in order to obtain theconditions for the first treatment step and, if need be, for the secondtreatment step. The first treatment parameters are preferably selectedin such a manner that no ablation of material or only a smallestpossible ablation of material occurs, since smoothing is effected inthis first treatment solely by remelting of the boundary layer down tothe first remelting depth. In smoothing and polishing plastics,conducting just the first step suffices to obtain excellent smoothingresults.

By utilizing continuous or pulsed energetic radiation, in particularlaser radiation, with a great pulse duration of ≧100 μs single ormultiple remelting of the boundary layer down to the first remelting isachieved. In contrast to this, in the second treatment step, pulsedradiation with a pulse duration of ≦5 μs is preferably employed togenerate the high intensities required for the combination remelting andmaterial removal process. In this second treatment step, the surface ispreferably only remelted down to a second remelting depth of maximally 5μm, whereas the greater first remelting depth of the first treatmentstep preferably lies between 10 and 80 μm. This first remelting depth ofthe first treatment step is dependent on the size of the macro-roughnessof the to-be-smoothed workpiece. The greater the to-be-smoothedmacro-roughness, the greater the depth of the first remelting depth hasbe selected in order to achieve sufficient leveling of themacro-roughness.

Furthermore, smoothing and polishing of the surface with energeticradiation should be conducted under a protective gas shroud. This mayoccur by treatment within a process chamber filled with protective gasor by feeding the protective gas to the surface areas being treated bymeans of a jet. Argon, helium or nitrogen can, for example, be used asthe protective gas.

Optimum smoothing results are yielded with the present method, if thesurface of the workpiece is remelted multiple times in succession in thefirst treatment step, preferably with a first remelting depth thatdecreases from remelting process to remelting process. The treatmentwith energetic radiation is, as in the second treatment step, conductedin a prior art manner by scanning the surface with the energetic beam.This scanning occurs in parallel paths, with the individual pathsdefined by the width of the single energetic beams partiallyoverlapping. In the multiple remelting of the surface, the treatmentdirection is preferably turned by an angle of, for example, 90° betweenthe single remelting processes in such a manner that the paths ofsuccessive remelting processes lie perpendicular to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

The present method is made more apparent in the following using apreferred embodiment with reference to the accompanying drawings withoutthe intention of limiting the scope or spirit of the inventive idea. Thefile of this patent contains at least one drawing/photograph executed incolor. Copies of this patent with color drawing(s)/photograph(s) will beprovided by the Office upon request and payment of the necessary fee.

FIG. 1 a shows a schematic sketch of the scanning of the to-be-smoothedsurface using a laser beam and FIG. 1 b shows an example of across-section of the laser beam;

FIG. 2 shows a rather schematic representation of an example of anoriginal profile of a surface including the results of various polishingconcepts;

FIG. 3 shows a comparison of surface profiles of an unprocessed milledsurface with a surface profile of the surface of following the firsttreatment step of the present method;

FIG. 4 shows an example of the treatment strategy for successiveprocessing a multiplicity of adjacent treatment sections;

FIG. 5 shows an example of the treatment strategy for retaining edges onthe to-be-smoothed surface;

FIG. 6 shows a schematic representation of an example of the differentremelting depths of the first and second treatment steps of the presentmethod;

FIG. 7 shows a representation comparing the measured surface profilefollowing the first and following the second treatment step;

FIG. 8 shows a photographic representation of a surface before smoothingusing the present method following the first treatment step andfollowing the second treatment step; and

FIG. 9 a and FIG. 9 b shows an example of smoothing a surface using thepresent method while retaining structures of significance.

WAYS TO CARRYING OUT THE INVENTION

FIG. 1 a shows very schematically the treatment process in the presentmethod by scanning the surface 1 of the to-be-treated workpiece 2 with alaser beam 3. The laser beam 3 is led in parallel paths 6, preferablymeandering, over a section 4 of the to-be-treated surface 1. The width 5(track width) of the individual paths 6 is given by the diameter 7 ofthe laser beam on the surface 1. In order to obtain a suited intensityor track width, this beam diameter 7 may, of course, be adapted with theaid of a interconnected optic. The laser beam 3 is led over the surface1 in the direction indicated by the arrow with a given scanningvelocity. Adjacent paths 6 overlap by selecting a track offset 8 whichis smaller than the track width 7. The length 9 of the individual paths6 can be predefined. It is, however, limited by the laser scanningsystem employed. Therefore, in order to treat large surface areas, amultiplicity of shown sections 4 have to be treated successively. FIG. 1also, indicates, by way of example a first remelting depth 10 down towhich the workpiece 2 is remelted in the first treatment step of thepresent method. In addition to a round beam cross section, another beamcross section 7 a, for example rectangular respectively linear can, ofcourse, also be employed as is indicated by way of example in FIG. 1 b.

In this first treatment step, a continuous or pulsed laser system withpulse lengths of >0.1 ms is utilized. The boundary layer of theworkpiece 2 is melted just so deep that the roughness present on surface1 is smoothed. This first remelting depth 10 is adapted to the originalroughness. Typical first remelting depths 10 lie in the range between 10and 100 μm. The greater the unevenness of the original surface 1, thedeeper the remelting has to be in order to permit the necessary volumecompensation. For example, greater first remelting depths are requiredfor milled surfaces than for abraded surfaces. Using a continuous laserbeam, respectively a pulsed laser beam with long pulse lengths, preventsevaporation of material from the surface 1 in this rough treatment ofthe first treatment step, permitting thereby carrying out the polishingprocess with substantially less energy than is the case withapplications in which the macro-roughness is removed. Furthermore, localoverheating in the molten bath which leads to material removal and toundesired molten bath movements and thus to deteriorating the surfaceroughness is largely prevented.

For this first treatment step of the present method, the beam source ispreferably a Nd:YAG laser, a CO₂ laser, a diode laser or an electronbeam source. The laser power lies in the range from 40-4000 W. The beamdiameter being 100-1000 μm, the scanning velocity is approximately30-300 mm/s and the track offset is selected between 10 to 400 μm. Theinteraction times with the surface lie preferably between 200 μs and 10ms. Passing surface 1, respectively the section 4 just undergoingtreatment, multiple times and turning the treatment direction by, forexample, 90° permits improving the first treatment step further.

FIG. 2 shows, as an example, in a very schematic representation asection of an abstracted original profile (FIG. 2 a) of a to-be-treatedsurface with macro-roughness with a height 11 of 10 μm and a width of,respectively a distance 12 of 300 μm (not shown to scale). Thesemeasures correspond to typical original dimensions of the originalroughness of a surface with a milling structure.

FIG. 2 c shows the effect of the first treatment step of the presentmethod in which the surface (previously: broken line; after: continuousline) is remelted to a remelting depth of approximately 10 μm. Remeltingthe material in this remelting step levels the macro-roughness.

In comparison, FIG. 2 b shows a result as obtained by flat removal ofsurface material in polishing. This example clearly shows that themacro-roughness cannot be completely removed by flat removal (20: areasof removed material).

FIG. 3 finally shows measured surface profiles of a to-be-treated,respectively treated, surface. FIG. 3 a shows a measured profile of anuntreated, milled surface where the macro-roughness is clearly visible.Following carrying out the first treatment step of the present method, aprofile of this surface is obtained, as depicted in FIG. 3 b. Thisfigure clearly shows definite smoothing of the macro-roughness after thefirst treatment step.

In the case of large to-be-smoothed surfaces, a multiplicity of sections4 of surface 1 depicted in FIG. 1 have to be treated successively withlaser radiation. In order that the boundaries, respectively thebeginning of the respective adjacent section (4) are not visible on thefinished workpiece, the treatment parameters are continuously changed,respectively changed in steps, down to the border of these sections 4 insuch a manner that the first remelting depth is reduced. FIG. 4 shows ina section an example of such a type of treatment strategy. In thissection, two treated sections 4 are adjacent to each other. In thetransition region between the two sections 4, the remelting depth 10 iscontinuously reduced in such a manner that there are no abrupt changesin smoothing in this transition region. Changing the treatmentparameters down to the border of section 4 can be achieved by defocusingthe laser beam, by reducing the power, for example using power ramps, byincreasing the feed rate, for example using feed rate ramps, or byvarying the position of the beginning, end, or turning points.

In polishing injection mold tools, it is essential that the edge at theseparation plane of the tool is not rounded, because this would lead toundesired formation of crests on the plastic parts produced with thetool. In order to prevent rounding at the to-be-retained edges of thesurface of the tool, a similar strategy can be used in carrying out thepresent method as in successively processing adjacent treatmentsections. The treatment parameters are changed toward the edge in such amanner that the first remelting depth decreases. The edge itself mustnot be remelted as this would always lead to rounding it. FIG. 5 showsthat of the two sections of surface 1 adjacent at edge 13, the firstremelting depth 10 decreases toward edge 13 so that no remelting occursat the edge 13 itself. This reduction of the remelting depth can beachieved by decreasing the laser power, respectively increasing the feedrate toward the edge 13.

After smoothing the surface in the first treatment step, the degree ofgloss is raised in a second treatment step using a pulsed laser withpulse lengths of <1 μs. By selecting a second remelting depth 14, whichis less than the first remelting depth 10, a very thin boundary layer of<5 μm is remelted and the remaining micro-roughness peaks 15 are removedby evaporating the material. This is depicted very schematically in FIG.6 showing the first remelting depth 10 and the second remelting depth 14as well as the remaining micro-roughness peaks 15 remaining after thefirst treatment step. A glossy surface is obtained with this secondtreatment step.

FIG. 7 shows a measured profile of a surface smoothed using the presentmethod. Depicted in the left part of the figure is the surface roughnessremaining after the first treatment step, and depicted in the right partof the figure is the surface profile after the second treatment step. Inthis representation, the reduction in the thickness of the lineindicates the substantial reduction of the micro-roughness to amagnitude of ≦0.1 μm remaining after the first treatment step.

Treatment in the second treatment step also occurs by scanning thesurface, for example on a meandering path. Typical treatment parametersfor the second treatment step are the use of a Nd:YAG laser or anexcimer laser with a laser power of 5-200 W and a scanning velocity of300-3000 mm/s with a beam diameter of 50-500 μm and a track offset of10-200 μm.

Finally, FIG. 8 is a photograph of a surface showing a region 16following milling, a region 17 following the first treatment step and aregion 18 following the second treatment step. The glossy surfaceyielded by the second treatment step is quite evident in this figurecompared to the smoothing of the first step, respectively compared tothe unsmoothed surface.

With suited selection of the treatment parameters, surfaces can also bepolished in such a manner that the structures of significance present ina surface are retained, undesired micro-roughness, however, is removed.By selecting the treatment parameters, in particular the first remeltingdepth, it can be set which structure of the surface are smoothed andwhich are to remain. Thus, for example, an eroded surface can bepolished to a high gloss while retaining the erosion structure and inthis way producing grained surfaces for injection mold tools, as isshown, for example, in FIG. 9 a and FIG. 9 b. FIG. 9 a shows an eroded,unpolished surface with corresponding structures of significance 19 andmicro-roughness 15. FIG. 9 b shows the same eroded surface aftersmoothing according to the present method. It is clearly visible thatthe micro-structures have been completely removed and but that thestructures of significance 19 are still present. Changing the treatmentparameters during treatment results in varyingly strongly smoothedstructures, and in this way different gray hues can be realized, forexample for creating inscriptions on a surface.

LIST OF REFERENCES

-   1 surface-   2 workpiece-   3 laser beam-   4 section of the surface, resp. treatment field-   5 track width-   6 path-   7 beam diameter-   7 a beam cross section-   8 track off set-   9 path length-   10 first remelting depth-   11 height of the macro-roughness-   12 width of, resp. distance, the macro-roughness-   13 edge-   14 second remelting depth-   15 micro-roughness-   16 untreated area-   17 treated area after the first treatment step-   18 treated area after the second treatment step-   19 structures of significance-   20 removed material

1. A method for smoothing and polishing a to-be-smoothed surface,comprising: a first treatment step comprising remelting theto-be-smoothed surface using energetic radiation while employing firsttreatment parameters at least once down to a first remelting depth whichis greater than a structural depth of to-be-smoothed structures of saidto-be-smoothed surface, wherein the using of energetic radiationincludes using continuous energetic radiation or pulsed energeticradiation with a pulse duration of ≧100 μs, such that said surface isremelted down to a first remelting depth of about 5 to 100 μm, and asecond treatment step comprising leveling micro-roughness remaining onsaid surface after said first treatment step by remelting themicro-roughness using said energetic radiation while employing secondtreatment parameters down to a second remelting depth, and byevaporating roughness peaks, wherein the second remelting depth is lessthan said first remelting depth.
 2. A method according to claim 1,including selecting said first treatment parameters so that no ablationof material occurs.
 3. A method according to claim 1, wherein the usingstep includes using pulsed laser radiation with a pulse duration of ≦1μs is employed in said second treatment step.
 4. A method according toclaim 1, wherein the remelting step includes remelting said surface insaid first treatment step down to a first remelting depth ofapproximately 10 to 80 μm.
 5. A method according to claim 1, wherein theremelting of said surface in said second treatment step includesremelting said surface down to a second remelting depth of maximally 5μm.
 6. A method according to claim 1, wherein the remelting stepincludes remelting said surface in said first treatment step multipletimes in succession.
 7. A method according to claim 6, wherein with eachnew remelting step, selecting said first remelting depth less deep thanin the previous remelting step.
 8. A method according to claim 6,wherein the remelting step includes leading said energetic radiation inparallel paths over said surface with successive remelting steps of asection of said surface being carried out with paths turned at an angle.9. A method according to claim 1, wherein treatment in said firsttreatment step occurs successively in a multiplicity of adjacentsections of said surface, with the treatment parameters being changedcontinuously or in steps towards a border of said sections in such amanner that said first remelting depth decreases to said border of saidsections.
 10. A method according to claim 1, wherein in order to retainedges on said surface, said first treatment parameters of said firsttreatment step are changed continuously or in steps in such a mannerthat said first remelting depth decreases toward said edges.
 11. Amethod according to claim 1, wherein the remelting step includes leadingsaid energetic radiation on one or a multiplicity of meandering pathsover said surface.
 12. A method according to claim 1, includingimpinging said surface with protective gas during said first and saidsecond treatment steps.
 13. A method according to claim 1, whereintreatment occurs with a beam cross section in form of a line or with arectangular beam cross section of said energetic radiation.
 14. A methodaccording to claim 1, further comprising preheating said to-be-smoothedsurface before remelting.
 15. A method according to claim 1, includingselecting said first treatment parameters so that structures ofsignificance of said to-be-smoothed surface are retained duringremelting.